Contamination of water supplies by heavy metals is a serious problem posing dangerous health hazards. Physical processes for heavy metals removal from solution are known in the art and include ion exchange methods using resins specific for individual heavy metals, and membrane filtration processes such as reverse osmosis and ultrafiltration using membranes selective for specific ions.
For instance, U.S. Pat. No. 4,578,195 describes a method using chelating ion-exchange resins. Methods of this and the spent brine, itself, becomes a waste problem.
Membrane filtration methods require high initial set-up costs due to the complexity of the apparatus involved. Furthermore, operating costs in terms of high wage manpower to monitor the relatively more sophisticated technology, and replacement costs for maintaining components needed for constant membrane cleaning and water pretreatment make this alternative an uninviting one.
Physical filtration techniques using steady-state or semifluidized filtration beds are also known. U.S. Pat. No. 4,438,000 discloses a filtration process using a semifluidized bed of particles immobilized by a porous retainer for the filter medium.
Methods based on fluidized bed filtration are also known for biological waste water treatment. For example, Soviet Pat. No. 604,287 discloses a column-type aeration tank for purification of sewage with biological activated sludge, and Soviet Pat. No. 714,770 describes a waste water purification method using bio-activated sludge which is subsequently phase separated in a bio-reactor.
Conventional chemical processes for precipitating heavy metals from solution include chemical oxidation at approximately neutral pH to convert soluble metal oxides into insoluble hydroxides, such as disclosed in U.S. Pat. No. 4,565,633. U.S. Pat. No. 4,764,284 describes a process of high pH alkaline precipitation of heavy metals as insoluble hydroxides in a reactor containing a fluidized bed of suitable bed material.
The problem with these methods is the difficulty of incorporating the general chemical concepts into workable, efficient and feasible chemical engineering systems. The current state of the art necessarily require multiple facilit-ies for activation, treatment and separation of heavy metals. Because multiple units are required, large areas are necessary, along with a relatively high manpower requirements for monitor-ing the separate systems. Additionally, the conventional systems require automation and control sub-systems which add to the complexity and cost of existing methods. All of these drawbacks have forced the industry to rely on electrochemical processes which are, currently, the only practical solution to large scale heavy metals removal.
Electrochemical methods for removal of high valency iron from solution are disclosed in U.S. Patent Nos. 3,926,754, 4,036,726, and U.S. Pat. No. 4,123,339. In these methods, insoluble iron matrices are generated with electrolysis using a suitable anode. Unfortunately start-up and operational costs for such electrolytic methods and operating costs are high.